It seems the debate between using a VMDK on VMFS or an RDM still rages when it comes to the question which one is better for performance.
The VMware team has published a lot of evidence in the past that the difference is very minor, in fact difficult to measure accurately and probably unperceivable to customers. Definitely not worth giving up the value of encapsulating a VM.
Published Dec 7, 2007 (on ESX 3.0.1)
Published Feb 13, 2008 (on ESX 3.5)
Let’s take a look at some new internal data for vSphere 5.1 that continues to validate that VMDK is the right default choice.
The Test Bed
This data was generated on vSphere 5.1 using the DVD Store test application running mySQL 5.5 which simulates an online e-commerce store. The scale out tests below were configured such that a number of DVD Store virtual machines were placed side by side on the Dell R910 (2x E5620) vSphere host. Additionally, a number of client virtual machines were required to simulate the high numbers of transactions.
--> The rest of the test is to be seen on the blog : http://blogs.vmware.com/vsphere/2013/01/vsphere-5-1-vmdk-versus-rdm.html